Cheap and easy digital tachometer?

I convert engines from weedeaters, leaf blowers, etc. for use on large model airplanes. I use an aftermarket electronic ignition on the engines. The ignition uses a Hall effect sensor and a magnet to trigger the CDI ignition. I would like to build a digital tachometer that would use that same trigger. They sell them for $25.00, but I thought it would be fun, and cheaper, to make one (or several) myself.

Ummm, tachs are handy to get the prop size correct and are seldom attached to the model airplane in flight. Instead they are part of the ground kit.

It might be tough to get lower than $25 but any one of the small microcontrollers (PICAXE, BasicStamp, Arduino) should be able to pull off this task. You might be able to pick off the existing ignition sensor, use another hall effect chip to sense the magnet or go optical off the prop. Add a serial 7 segment LED or LCD display and you would be revin.